Wales have turned to Bristol-born Kohn following a tournament build-up ravaged by second-row injuries, with Alun-Wyn Jones, Luke Charteris, Bradley Davies and Ian Evans all currently sidelined.
Kohn, 31, qualifies for Wales through his grandfather, who hails from the Rhymney Valley.
Wales confirmed that Kohn would link up with the squad on Sunday evening ahead of the following Saturday's Six Nations opener against Ireland in Cardiff.
Kohn joined Quins in 2006 and he has since made more than 130 appearances for the current Aviva Premiership champions.
He becomes the third uncapped lock in Wales' Six Nations squad, joining Ospreys forward James King and Newport Gwent Dragons second-row Andrew Coombs.
"We looked at Olly when we initially selected the Six Nations squad, and we have brought him in now as we believe he will be an experienced addition," Wales' interim head coach Rob Howley said.
"He has been performing very well for Quins and has been a feature in their pack.
"He is a strong ball-carrier and dominant at the set-piece, and it will be good to see him in the international environment."
Howley is due to announce his team for the Ireland game next Thursday.
Jones, Charteris and Davies are all unavailable, while Evans has been battle to overcome a knee injury to be fit in time.
Howley has an option of switching former Wales captain Ryan Jones from the back row to second row, although he is currently recovering from a dislocated thumb suffered on Heineken Cup duty against Leicester 12 days ago.
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